I have a Macintosh 128k and I want to connect its screen with a Raspberry Pi. Is it possible? How? Can I take some parts of another cathodic TV like composite video?
It will be difficult, since little to no standards were employed. It was a cutting edge device at the time, almost every part of it was made for purpose, since there weren't all that many established common standards that applied back then.
No matter what you do, it will involve some degree of reverse engineering. And the more schematics and other technical documents you can get hold of, the better.
If you are lucky, you can patch into the system at a relatively high level digital interface. You'll just have to work out what that point is, and how it works. The saving grace here might be that the electronic components are "good old-fashioned" logic DIL chips with large 1/10th inch pitch running at low speed with no software, so it would be relatively easy to use a dirt cheap logic analyser to watch signals on a PC and see what's going on. Then sever those connections on the PCB and feed in your own.
The only other way would be to scrap most or all of the electronics altogether and control the CRT directly. It might not be that hard to get something going, assuming you don't electrocute yourself, but the picture would most likely be bad. You could in theory use some driving electronics from some other CRT, but chances are you'd end up with the same or more of a headache. Ultimately, you want to copy the way it is driven. If you go this route, I'd suggest an oscilloscope and life insurance.
In short, you're best off finding schematics and whatever other technical docs you find, and understand as much of it as possible, then replace the circuitry at the highest possible level of abstraction, whatever that might be. Saying that, a 5 minute look at its schematic might tell you a great deal about what approach to take... if you can find it.